The requirements for VPCs referenced by x-aws-vpc are documented somewhere.
I can find no documentation about the requirements for VPCs that one wished to use with Docker for ECS
I found this Shows "VPC should have at least 2 associated subnets in different availability zones" but my VPC has 2 subnets in different AZ's · Issue #864 · docker/compose-cli · GitHub which strongly suggests that VPCs are validated by searching for a subnet in the named VPC but I can find nothing related to requirements for the subnets. The title of the issue above provides a hint but it does not purport to be a complete description of the requirements.
I am working in an account that does not have a default VPC. I created a VPC and nothing else. When I pointed my context at my AWS account and ran “docker compose up” I got this message: InvalidVpcID.NotFound: The vpc ID ‘vpc-XXX’ does not exist. It definitely does exist but I suspect it does not meet some (as far as I can see, undocumented) requirements for the VPC.